He killed a bunch of Nightsisters, a rancor, and the Nightsister's Matriarch, killed several magnaguards as a clone trooper, in melee combat, and killed Vader's personal droid bodyguards on a dark side nexus despite being light side.
He also stopped a siege on cloud city, slaughtering more then a hundred stormtroopers and 6 dark troopers single-handedly. (I mean, Lobot and Lando helped near the end, but it was mainly just him)
The dude is decent, you just need to check the respect thread and also look up some of his feats.
If you don't consider those decent enough showings, then I dunno what to tell you.
And the whole rancor-killing thing happens in between Vjun and Mustafar, thats where X2 learns about the wookie clones, and how X2 finds out that X1 has his base on Mustafar.
It doesn't contradict anything, I'd put it on path with the PSP version, nothing would have changed had it been in the PSP version besides an extra level.
Just as well, its the final mission of the DS version if I remember correctly, which makes it canon to the main story as whole, as afterwards, X2 fights X1 on mustafar.